(1) Field of the Invention
The present invention relates generally to the utilization of thermoplastic blended compositions of soy meal and natural rubber for the preparation of thermoset biodegradable elastomers.
(2) Description of the Related Art
Non-biodegradable plastics, which are mainly from petroleum resources, are becoming a burden on local landfill. Non-biodegradable plastics waste also increases the pollution of greenhouse gas emissions when burned, which causes the present global warming problem (Kerr, R. Science. 2000, 288 (5466), 589) and (Patel, M. Ph.D. Thesis, Closing carbon cycles: carbon use for materials in the context of resource efficiency and climate change, Utrecht University, Netherlands, December 1999). Therefore, biodegradable polymers have attracted much attention.
At present, biodegradable polymers include synthetic polymers, such as polylactic acid (PLA), biopolymers produced by microorganism, such as polyhydroxybutyrate (PHB) and natural biopolymers, such as starch and protein. The former two kinds (PLA and PHB) of biodegradable polymers are costly (Okada, M. Prog. Polym. Sci. 2002, 27, 87). Special attention is paid to natural biopolymers because they are abundant and low cost.
Soy protein, as the most abundant protein in the world, is classified into soy protein isolate (SPI), soy concentrate (SC), soy flour (SF), and soy meal (SM) their costs being ˜$1.50; ˜$0.80; ˜0.20; ˜$ 0.8 per pound respectively. The cheapest soy protein is SM (0.18 US$/kg) (Information from Michigan Soybean Promotion Committee, January, 2003), which contains a minimum of 44% protein. However, this protein has poor processability because protein content is low and non-thermoplastic content is high.
Usually, soy protein isolate containing at least 90% protein has been used to prepare biodegradable materials, as mentioned in several patents (U.S. Pat. No. 6,632,925 to Zhang et al; U.S. Pat. No. 5,710,190 to Jane et al; U.S. Pat. No. 5,523,293 to Jane et al and U.S. Pat. No. 5,397,834 to Jane et al). In these processes, SPI, plasticizers such as water and glycerol, and other biodegradable polymers such as polylactic acid (PLA), were mixed and fed into an extruder to prepare pellets or foam. Rayas and Hernandez (U.S. Pat. No. 6,045,868) used wheat flour, glycerol as plasticizer, and formaldehyde solution as the cross-linking agent to prepare protein film. Bassi et al (U.S. Pat. No. 5,665,152 to Bassi et al) disclosed a composition of blends of grain protein with starch. The blends were said to be useful for molded plastics, but became brittle and sensitive to water. Sun et al (U.S. Pat. No. 6,716,022 to Sun et al) used wheat straw fiber, soy flour and water to compression-mold livestock feed supplement containers. These containers also became brittle at ambient conditions. As reported, (Zhong et al., Polymer, 2001, 42, 696) soy protein isolate/polycaprolactone blends (50 g/50 g) were brittle (σb=5-7 MPa, εb=2-3%), and became tough (σb=25-27 MPa, εb=10-12%) after cross-linking using 5 wt % of methylene diphenyl diisocyanate. However, the two raw materials are expensive.
However, there still exist the following main drawbacks for protein-rich biodegradable materials that has not been overcome by these patents. (1.) Protein becomes brittle at dry state due to the loss of water; (2.) Protein is difficult to process due to a high melting temperature. Therefore, a large amount of plasticizer is needed for processing; (3.) Protein is sensitive to water and has weak mechanical properties in a wet state; (4). At a high relative humidity state, plasticizers, such as glycerol or sorbitol, can penetrate out of protein matrix, and form liquid-like drops on the surface of a film. This phenomenon is called as “leaching”. The leaching phenomenon limits the usage of natural biopolymers. (5). When the content of protein in polymers blends reaches up to 50 wt % or higher, the mechanical properties of the blends decrease dramatically and can not satisfy customers' requirement. (6). Soy protein isolate which contains around 90% protein is expensive (˜3.3 US$/kg) for plastic applications.
The price of natural rubber (NR, 1.16˜1.43 US$/kg for Standard Malaysian Rubber (SMR)) Information from http://www.rubbercommerce.com/priceindices_unreg.jsp#, update Jul. 23, 2004 is cheaper than that of low density polyethylene (LDPE, 1.47˜1.69 US$/kg Information from http://www.plasticsnews.com/subscriber/resin/price1.html, update Jul. 23, 2004. In addition, natural rubber is a bio-based and biodegradable natural polymer while LDPE is not. Therefore, natural rubber has potential for developing low cost biodegradable polymers. Ezoe (U.S. Pat. No. 5,523,331 to Ezoe) blended natural rubber (67 wt %) and starch etc., in an extruder, and then vulcanized the blends to prepare biodegradable articles. But it was difficult to reduce the content of natural rubber below 50% because starch was not plasticized, making the blends difficult to process if starch content reached a high level. Japanese Patent, P2001-288295A, to Katuaki directly blended natural rubber (60 wt %) and corn protein to prepare biodegradable thermoplastic in an extruder, but the blends had leaching problems because of the existence of the large amount of plasticizers, such as glycerol. In addition, the blends were very sticky because the natural rubber was not vulcanized. To improve the flexibility of thermoplastic starch, 2.5-20 wt % of natural rubber was blended into a starch matrix in an intensive mixer at 150° C. (Carvalho, A. J. F.; Job, A. E.; Alves, N.; Curvelo, A. A. S.; Gandini, A., Carbohydr. Polym. 2003, 53(1), 95). Results revealed a reduction in tensile strength and an improvement in flexibility. These blends can have stickiness problems, although the natural rubber content is low.
While the related art teach blends of natural rubber and biopolymers, there still exists a need for improved biodegradable thermoset blended compositions.